Being able to test more than one device at a time on a given tester is a simple and effective means of lowering test time and improving throughput, provided the hardware and software allow it. There are generally two different methods for testing multiple devices simultaneously on a tester.
The first method is referred to as dual-site testing. In this method, the hardware is split evenly into an X quadrant and a Y quadrant, and each quadrant is controlled by either the primary or a secondary controller. Dual site testing is basically just two test programs running simultaneously on the same TESTER, each having its own controller and hardware. Since there is only one high speed clock on the TESTER, special attention must generally be paid when using clocks to allow the two test programs to synchronize.
The other method of simultaneous testing is referred to as multi-site testing. In this method, the test hardware is split up by the test software. This means that more than two devices can be tested if the hardware allows. The software generally allows up to 32 devices to be tested in the multi-site mode, although, for most devices, hardware limitations limit the number of devices to generally 8 devices. Multi-site testing is generally preferred over dual-site for a number of reasons. After a multi-site test program is written, it can easily be modified to test a different number of devices if needed, while dual-site testing is limited to only two devices. Also, because dual-site testing utilizes two separate controllers, it does not allow for as accurate control and timing of the test program, compared to a test program controlled by a single controller.
Currently, the automatic testing (AT) procedures for handling a given lot (or batch) of semiconductor units are yield-dependent. Many varying factors contribute to low-yield, and hence, a re-screen of failed units is usually performed. Retesting of reject units is known for re-binning a percentage of reject units into good units. A conventional “manual” retest method (generally referred to as the “Manual rescreen”) comprises retesting all rejected units after all units in the lot have completed the test. Operators generally retrieve units binned as rejects, and manually reload these units into the handler for retesting. The manual rescreen method generally results in wasted time waiting for:    a) trailing units in the lot to be unloaded/sorted;    b) the handler to determine a clean empty state;    c) the operator to close the current segment (requires counting and handling of units);    d) the operator to initialize a new segment for the re-screen (requires counting and handling of units in addition to other paperwork/actions);    e) the operator to manually load the units into the input of the handler;    f) the handler to initialize its modules coming from a stop state, and    g) the handler to load the units into the contact site(s) to be tested.
Although generally effective for re-binning a percentage of reject units into good units, potentially productive test time is wasted by the required operator handling time, as well as time for the handler to index the units for retest.
Another known retest method for multi-site testers comprises the “auto-reprobe” method which results from the capability to re-insert the failing units. Auto-reprobe is performed by issuing a retract and contact of the contact site plunger mechanism. Primarily designed to improve miscontact and reduce overkill, auto-reprobe is generally an improvement as compared to the manual rescreen method. However, auto-reprobe is known to work well for only for devices with long test times and lower parallelism on the load boards. The inherent problem with the auto-reprobe method is that it reduces multi-site efficiency (MSE) while trying to improve miscontact. Specifically, good units are replunged with the bad devices. When devices run comparatively short test times and high parallelism, the adverse affect on MSE becomes more severe. As a result, for devices with short test times (e.g. <5 secs) and higher multi-site solutions (e.g. ≧8), auto-reprobe may result in no test efficiency improvement over the manual rescreen method.
Variants of known retest methods include auto-reprobe only during a rescreen segment. This variant does not generally eliminate the losses described above. This variant merely reduces the possibility of a 2nd manual re-screen.